There are 4 wrenches just unpacked from a box. They all look the same. There might be one defective item which is either heavier or lighter. Fortunately there is a fifth one that is standard. If you can use a scale to weigh only 2 times, can you find out:

You can weigh the whole lot of spanners and then weigh one. This doesn't neccessarily work but if you pick a normal one, you can divide the whoile mass by 5 and see if you come up with the weight of the single one you weighed!